Bridge Deck Reinforcing Steel Cover Depth Prediction Using GPR

With increasing traffic volume on the Dan Ryan Expressway (I-90/94) bridge in Chicago, early detection and quantification of possible internal flaws and deterioration is needed. Using a reliable measurement tool, informed decisions can be made about appropriate and timely repair and rehabilitation actions. In this study, six lanes, with nine sections each, of the Dan Ryan bridge deck were tested using ground penetrating radar (GPR). An image processing technique of the GPR data was then used to detect the reinforcing bars (rebar) and to estimate their cover depth profile. To estimate the cover depth profile, calculated concrete dielectric constants were utilized. The results of the analysis showed a variation of the rebar cover depth within and between the surveyed sections. The rebar cover depth of the surveyed lanes was mostly greater than 64mm (2.5in). The southbound middle lanes, #2, #3, and #4, had low cover depth, especially lane #3. This lane had a high percentage (44%) of a cover depth lower than 64mm (2.5in); 7% less than 50mm (2in). Locations determined to have extremely low rebar cover depth, less than 38mm (1.5in), showed spalling. The results of the GPR analysis were verified using limited ground truth cover depth measurements. The variation in the cover depth profile could be caused by rebar floating during construction.